Patent Document 1 discloses an engine which combusts air-fuel mixture in a combustion chamber by autoignition in a partial load region. This engine promotes autoignition of air-fuel mixture by leaving hot burned gas in the combustion chamber in an operation region on a low load side in the partial load region. Furthermore, in the engine, in an operation region on a high load side in the partial load region, cooled burned gas is introduced into the combustion chamber such that autoignition is less likely to occur, and a spark plug performs ignition immediately before the compression top dead center.
In addition, Patent Document 2 describes an engine in which autoignition of air-fuel mixture is performed twice at different times for the purpose of reducing combustion noise in a partial load region. In the engine, fuel is injected toward a region outside a cavity of a piston in a compression stroke. Accordingly, autoignition of air-fuel mixture formed outside the cavity is performed at or near the compression top dead center. In addition, in the engine, after this autoignition, fuel injection for the second time is performed. Accordingly, air-fuel mixture is formed in the cavity, and the air-fuel mixture in the cavity is combusted by autoignition.